Cloud computing is perceived as the technology enabling an on-demand provisioning of highly reliable virtualized resources such as compute, storage and network, which can be all-time accessed from everywhere.
Promising achievements in terms of virtualization technologies made in the last decade, which enabled data centre owners to better utilize their infrastructure have become instrumental drivers of the success we are now witnessing around cloud computing.
Cloud computing is known as a layered paradigm. Depending on the service being offered it can be referred as:                Software-as-a-Service (SaaS): Enables consumers to use applications that run on a cloud computing network.        Platform-as-a-Service (PaaS): Enables consumers to create and deploy applications that make use of the cloud computing network.        Infrastructure-as-a-Service (IaaS): Enables consumers to access processing, storage, networks and other fundamental resources in order to deploy and run arbitrary software.        
Content Delivery Networks (CDNs) provide mechanisms and network infrastructure that enable service providers to improve the accessibility of their content to end-users (customers).
A key component in a CDN network is a cache that is typically located close to the end-user and is in charge of caching content, thus reducing the time to fetch that content.
Another important component of a CDN is the function that finds out where a certain end-user should fetch the requested content. To determine that, the CDN will use the IP address of the end-user, apply some logic that takes into account where the CDN has the requested content cached, and then finally, redirect the client to the best located cache. CDNs are good at serving content (web pages and bulky video and audio files) but do not provide mechanisms to serve cloud service (e.g. SaaS). CDNs can be seen as smart storage boxes, servers, but which are lacking many computing capabilities.
Cloud computing on the other hand does not provide this flexibility, especially not if the cloud customers are moving around such as truck drivers, sales persons, repair men etc that are depending on mobile broadband to access its firm's cloud services.